- other students to practice with (new friends who share an interest!)
- a teacher you can ask questions and build a relationship with (if they're a native speaker, this can be motivating)
- you're forced to listen, speak, read, and write, often before you feel ready
I started with language courses, and lucked out with a great teacher and fun students. I spent lots of time with the language and advanced really quickly, and my classes were perfect places to try out my new knowledge. I still remember one day we were having some natives over to interview, and I racked my brain to come up with interesting questions; one of them was 'What's in your fridge at home?'. My point is that my class was a great place to try out what I was learning at home on my own time. Also lots of fun memories.
I'd agree on asking questions about grammar; it's possible to study the language instead of learning it. What I mean is that studying and having the skills - speaking, writing, etc - are two very different things.
As far as learning grammar, this cycle proved extremely effective for me:
- Learn about grammar in class
- Learn about grammar on my own (e.g. compare English and Russian grammar )
- notice new grammar in whatever I was listening or reading
Jim Scrivener (an English teacher and teacher trainer) writes this about grammar in 'Learning Teaching' (p. 253):
"It seems likely that learners have to do a number of things to be able to start making any new grammar item part of their own personal stock of language.
They probably need to have _exposure_ to the language; they need to _notice_ and _understand_ itms being used; they need to _try using_ language themselves in 'safe' practice ways and in more demainding contexts; they need to _remember_ the things they have learnt." 
So it follows from this that learning needs to be a very rich process - bursting at the seams with context and emotions and lots of words; where Duolingo and the others are more similar to a Greek salad: not bad, but you're not getting everything you need.
So use language learning apps, but make sure your language learning diet is giving you everything: lots of exposure, time to notice and understand, opportunities to practice - from easy to demanding, and plenty of review.
If you have a healthy diet - lots of exposure to the language - then you'll review grammar structures and words as a natural part of your listening/reading.
2: https://books.google.com.ua/books?id=vPWdBgAAQBAJ&pg=PT8&dq=... (different book, same quote)